The Treatment Section continues to investigate predictors that might be useful for patient-treatment matching to improve the delivery of appropriate treatments to individual patients. Marriage has been cited as a protective factor against drug use, but the relationship between marriage and drug use has not been explored longitudinally during addiction treatment. In a recently completed project, we assessed individual trajectories of substance use during treatment as a function of marital status and perceived closeness of the marital relationship. A parallel-process growth model was used to (1) estimate the rate of change in percentage of cocaine-positive and heroin-positive urine samples, and (2) examine the relationship between marital status and drug use trajectories over 35 weeks, during and after treatment. Percent days of use for both drugs were lowest for married participants across all time points. Among married participants, reporting a close relationship with one's partner predicted less cocaine and heroin use. These findings suggest that being married and having a close relationship with one's spouse are associated with better outcomes over time. The causal nature of the association is suggested by previous research that has demonstrated the effectiveness of couples therapy as an adjunct to methadone maintenance.